1. Field of the Invention
The present invention relates to an optical encoder and to a lens fixing mechanism of the optical encoder. In particular, the present invention relates to an optical encoder that detects a track on a scale by forming an image of the track on a light-receiving element by using an imaging optics including a plurality of lenses and an aperture plate, and to a lens fixing mechanism that is preferably used for such an optical encoder.
2. Description of the Related Art
FIG. 1 illustrates an example of an optical encoder including a detection head 1 having an imaging optics. The optical system forms an image 3 of a scale 2 on a light-receiving element 4. In particular, Japanese Unexamined Patent Application Publications Nos. 2011-13093 and 2011-59055 each describe an optical encoder including a double telecentric optical system in which lenses 5 and 6 are disposed on the two sides of the aperture plate 7. Such an encoder has an advantage in that distortion of an image due to variation in the gap between the scale 2 and the lens 5 on the front side and the gap between the lens 6 on the rear side and the light-receiving element 4 is small. In FIG. 1, an arrow X indicates the longitudinal direction of the scale, and an arrow Z indicates the optical axis direction.
A pair of identical optical systems may be parallelly arranged in an optical encoder having a two-track scale. In this case, a lens array is used in order to reduce the size of the optical encoder and simplify the components of the optical encoder, as described in Japanese Unexamined Patent Application Publications Nos. 2012-32295 and 2008-3014. In order to reduce the number of lenses and to eliminate optical aberrations by using only two lenses, aspherical lenses are used. FIG. 2 is a schematic view of an optical encoder including a double telecentric optical system including a two-track scale 12, an entry-side lens array 13, and an exit-side lens array 14. In FIG. 2, an arrow X indicates the longitudinal direction of the scale, and an arrow Y indicates the lateral direction. As described in Japanese Unexamined Patent Application Publications Nos. 2008-3421 and 2008-3466, a plastic is preferably used as a material of a compact high-accuracy aspherical lens. Hereinafter, it will be assumed that lenses are made of a plastic.
In order to achieve required optical characteristics, it is necessary that these optical elements be assembled with a relative positional accuracy of about a hundred micrometers.
Moreover, in an optical encoder having a two-track scale, in order to detect each track of the two-track scale by using two independent optical systems disposed in one detection head, it is necessary that the axis of the two-track scale 12 in the lateral direction (Y direction) and a line connecting the optical axes of the two optical systems be parallel to each other with high accuracy. Deviation from parallel (yawing) causes a decrease in the signal detection efficiency of an incremental encoder and positional error between the two tracks (inter-track error).
In a double telecentric optical system illustrated in FIGS. 3A and 3B, it is required that the focal points of both lenses be positioned at the aperture plate 7 with high accuracy, and it is necessary that the entry-side lens array 13 be fixed to a metal housing 15 of the detection head with high accuracy.
The lens arrays 13 and 14 may be fixed to the housing 15 by using a method including steps of, for example, forming bosses 13A and 14A on the lens arrays 13 and 14 so as to be located on a line connecting the optical axes of the lens arrays 13 and 14, applying an adhesive to the bosses 13A and 14A, and inserting the bosses 13A and 14A into boss-insertion holes 15A in the housing 15. With this method, as suggested in Japanese Unexamined Patent Application Publication No. 2004-93556, a surface of the lens (5 or 6) and the bosses 13A or 14A can be integrally formed by using a single die, so that the optical system can be easily manufactured with high accuracy. FIG. 3A is a schematic sectional view illustrating the entry-side lens array 13 and the housing 15, which are bonded to each other.
However, when the ambient temperature changes, a stress is applied to an adherend surface 13B of the entry-side lens array 13 as illustrated in FIG. 3B, because a plastic material of the lens arrays 13 and 14 has a coefficient of linear expansion (about 60×10−6 mK−1) that is about three times larger than that of a metal material of the housing 15, such as aluminum (23×10−6 mK−1). Moreover, because a stress is not generated on a side of the entry-side lens array 14 opposite to the adherend surface 13B, the entry-side lens array 13 receives a bending force indicated by arrows A. As a result, the surfaces of the lenses 5 of the entry-side lens array 13 become deformed as illustrated by broken lines, thereby causing a decrease in the contrast or distortion of an image, which may lead to reduction in the efficiency of an encoder signal and reduction in accuracy. The same applies to the exit-side lens array 14.
Japanese Unexamined Patent Application Publication No. 2008-3466 describes a technology for reducing variation in the curvature of a lens due to temperature variation by mechanically restraining a plastic lens by using a metal plate. However, this technology is not sufficiently effective.